Increasing daily light from 8 to 16 h per day in cattle markedly stimulates secretion of prolactin and testosterone while reducing secretion of glucocorticoids. There is a coincidental increase in body growth, acceleration in onset of puberty and lactational performance. The overall goal of this proposed research is to achieve a better understanding of the role of pineal gland and of melatonin in mediating photoperiod-induced changes in secretion of prolactin, testosterone and adrenal glucocorticoids in a anonseasonal breeding species, cattle. These hormones may be related to the photoperiod-induced changes in growth rates, onset of puberty, and lactation. Furthermore, since humans are reproductively active throughout the year, we believe cattle could be a good model for these studies. For this purpose, we will determine if pinealectomy suppresses the ability of increasing light to stimulate secretion of prolactin and testosterone and to inhibit secretion of glucocorticoids. We also plan to determine if pinealectomy affects seasonal changes in secretion of prolactin. In an effort to determine if melatonin blocks photoperiod-induced changes in secretion of prolactin, testosterone and glucocorticoids, we will infuse this indole subcutaneously for 8 h each day into animals maintained on 16 h of light per day. In other experiments, melatonin will be infused for 8 or 16 h per day into normal, pinealectomized or blind animals maintained on continuous light to determine if melatonin will mimic photoperiod-induced changes in secretion of prolactin, testosterone and glucocorticoids. As a secondary objective, the capacity of anterior pituitary explants from bulls pretreated daily with melatonin to bind dopamine and to release prolactin in vitro will be determined. Finally, an attempt will be made to determine if in vitro incubation of anterior pituitary cells with melatonin for 8 vs. 16 h each day will induce changes in secretion of prolactin that mimic the photoperiod-induced changes in vivo.